Currency's are far more than symbolic items. The Eurozone has been a prime example of how richer countries can mess up smaller ones. Smaller nations like Greece and Portugal which in other scenarios would have devalued their currency such as during the financial crises of 2008/09, had to instead contend with having an extremely high valued currency (EURO) which made exporting products and thereby boosting their economy difficult for them.
High value currency means your products become more expensive and theoretically less competitive in the global market but importing prodcuts become cheaper. This works great for countries like Germany, France which import cheap raw materials, turn them into products using their advanced technological base, before exporting and selling them on the global market for a profit using their brand (German cars are prestigious in large parts of the world for example). However countries with small industrial bases and low competitiveness such as Greece were screwed over by this.
In a hypothetical Muslim currency, countries such as Saudi and Qatar would lobby to make the currency value higher as oil and gas are only produced by a select few countries in the world in significant quantities. A high valued currency meanwhile would make the import of luxury items cheaper for the rich citizens of these countries. Meanwhile countries like Pakistan and Uzbekistan would be lobbying to reduce the value of the currency so as to boost their under pressure exports, especially as the numebr of rich indiviudals who can afford high luxury products in such countries are more limited.
Overall it would be a shi*storm, as nobody would be able to come to a conclusion due to hugely varied socio-economic strucutres in societies across the Muslim world.
